ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus GmbHGöttingen, Germany10.5194/acp-9-3147-2009The influence of traffic and wood combustion on the stable isotopic composition of carbon monoxideSaurerM.1PrévôtA. S. H.1DommenJ.1SandradewiJ.1BaltenspergerU.1SiegwolfR. T. W.11Paul Scherrer Institut, 5232 Villigen PSI, Switzerland150520099931473161This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from http://www.atmos-chem-phys.net/9/3147/2009/acp-9-3147-2009.htmlThe full text article is available as a PDF file from http://www.atmos-chem-phys.net/9/3147/2009/acp-9-3147-2009.pdf

Carbon monoxide in the atmosphere is originating from various combustion and
oxidation processes. Recently, the proportion of CO resulting from the
combustion of wood for domestic heating may have increased due to political
measures promoting this renewable energy source. Here, we used the stable
isotope composition of CO (&delta;<sup>13</sup>C and &delta;<sup>18</sup>O) for the
characterization of different CO sources in Switzerland, along with other
indicators for traffic and wood combustion (NO<sub>x</sub>-concentration, aerosol
light absorption at different wavelengths). We assessed diurnal variations
of the isotopic composition of CO at 3 sites during winter: a village site
dominated by domestic heating, a site close to a motorway and a rural site.
The isotope ratios of wood combustion emissions were studied at a test
facility, indicating significantly lower &delta;<sup>18</sup>O of CO from wood
combustion compared to traffic emissions. At the village and the motorway
site, we observed very pronounced diurnal &delta;<sup>18</sup>O-variations of CO
with an amplitude of up to 8&permil;. Solving the isotope mass balance equation for
three distinct sources (wood combustion, traffic, clean background air)
resulted in diurnal patterns consistent with other indicators for wood
burning and traffic. The average night-time contribution of wood-burning to
total CO was 70% at the village site, 49% at the motorway site and
29% at the rural site based on the isotope mass balance. The results,
however, depend strongly on the pure source isotope values, which are not
very well known. We therefore additionally applied a combined
CO/NO<sub>x</sub>-isotope model for verification. Here, we separated the CO
emissions into different sources based on distinct CO/NO<sub>x</sub> emissions
ratios for wood combustion and traffic, and inserted this information in the
isotope mass balance equation. Accordingly, a highly significant agreement
between measured and calculated &delta;<sup>18</sup>O-values of CO was found
(<i>r</i>=0.67, <i>p</i><0.001). While different proxies for wood combustion all have
their uncertainties, our results indicate that the oxygen isotope ratio of
CO (but not the carbon isotope ratio) is an independent sensitive tool for
source attribution studies.